The invention relates to a method for controlling the superheat temperature of the refrigerant in an evaporator arrangement of a refrigeration system or heat pump system, in which the evaporator arrangement, a compressor arrangement, a condenser and a controllable expansion valve arrangement are arranged in succession in a closed circuit and the superheat temperature is controlled in dependence on a comparison between desired and actual values, the desired value of the superheat temperature being varied automatically in dependence on the difference from a reference value of a periodically determined function of a number of sampled values of a temperature of the refrigerant with the aim of providing a stable control of the superheat temperature.
A method and a device of that kind are known from DE 37 13 869 C2. In that publication, the superheat temperature desired value is automatically matched to the particular operating conditions, such as refrigeration output, supercooling, evaporation temperature and so on, regardless of the type of refrigerant. Matching is effected in that the superheat temperature desired value is changed abruptly in dependence on the rate of change of the superheat temperature and additionally also whenever the superheat temperature falls below or exceeds predetermined limit values.
The superheat temperature can be identified as the difference in the temperature of the refrigerant at the output of the evaporator arrangement, that is, the vapour temperature, and its temperature at the input to the evaporator arrangement, or directly (as true superheat) as the difference between the refrigerant temperature at the output of the evaporator arrangement and the evaporation temperature.
The temperature of the refrigerant at the output of the evaporator arrangement is dependent on the extent to which the evaporator arrangement is filled with refrigerant and on the ambient temperature, for example, the air temperature, whereas the temperature of the refrigerant at the input of the evaporator arrangement is dependent on the pressure in the evaporator arrangement. In practice, the pressure at the output of the evaporator arrangement is frequently used as a measure of the evaporation temperature.
The measuring signal of the superheat temperature and its rate of change therefore change also whenever there are changes in the evaporation temperature. In the case of refrigeration or pump systems in which the temperature of the refrigerant is unstable at the input of the evaporator arrangement, the superheat temperature is also unstable, even when the refrigerant vapour leaving the evaporator arrangement has an adequate superheat temperature. In refrigeration systems for supermarkets, current practice is often to use several evaporator arrangements connected to a compressor arrangement having several compressor stages. This means that the output pressure of the evaporator arrangement is able to fluctuate wildly. Furthermore, in the method according to DE 37 13 869 C2 the superheat temperature can be set to a value that is too high, so that the evaporator arrangement is not filled with refrigerant to the optimum and efficiency becomes too low. In addition, in the event of brief, rapid fluctuations in the superheat temperature or on receipt of brief electrical interference signals on account of the differentiation applied in the known method, there are large variations in the desired value, even though the fluctuations or interference pulses have already disappeared again at the time at which the change in desired value is taking effect.
The digital differentiation used in that publication then requires a plurality of sampled values in order to determine the rate of change in the superhe at temperature with sufficient accuracy and speed. This in turn presupposes a high memory capacity in the microprocessor used for that purpose.